Pull up revision 1.214 (requested by yamt in ticket #251):
fix problems related to loopback interface checksum omission. PR/29971.
- for ipv4, defer decision to ip layer as h/w checksum offloading does
so that it can check the actual interface the packet is going to.
- for ipv6, disable it.
(maybe will be revisited when it implements h/w checksum offloading.)
ok'ed by Jason Thorpe.